The United States is facing "one of the largest" West Nile outbreaks in history.
The U.S. Centers for Disease Control said so far, 1,118 illnesses have been reported -- half of them in Texas.
"We're in the midst of one of the largest West Nile outbreaks ever seen in the United States," Dr. Lyle Petersen, a CDC official, said.
He added that many illnesses have never been reported before so early. Most infections are usually reported in August and September, so numbers could rise.
This summer 41 people have already died from the mosquito-born disease. That's four times the usual number of cases for this time of year.
Experts blame a mild winter, early spring, and hot summer temperatures for the spike in the mosquito population.
Only about one in five people infected with West Nile virus actually get sick.
Symptoms of West Nile fever include fever, headache, tiredness, body aches, and sometimes a skin rash.
More severe cases can lead to neck stiffness, disorientation, tremors, muscle weakness, and more.
For more on the West Nile virus, symptoms, and prevention, visit the CDC West Nile Virus page.